In an image forming apparatus using an electrophotographic technology, two systems have been known in the conventional art to develop an electrostatic latent image formed on an image carrier. One is a one-component developing system that uses only toner as a developer, and the other is a two-component developing system that uses both toner and carrier.
In the one-component development system, a toner-supporting member and a regulating plate pressed against the toner-supporting member are generally used. The film thickness is regulated while the toner on the toner-supporting member is pressed by the regulating plate, whereby thin toner layer of a predetermined electrostatic charge can be formed. An electrostatic latent image is developed on the image carrier with this thin toner layer. This method is characterized by excellent dot reproducibility and is capable of providing uniform images with the minimum irregularity. This method also simplifies the structure, downsizes the apparatus and reduces the production cost. However, a heavy stress is applied to the toner in the regulating section made up of a toner-supporting member and a regulating plate pressed against the toner-supporting member. This will degenerate the toner surface, and the toner and external additive agent will attach to the toner regulating member and toner-supporting member surface, with the result that the electrostatic charge of toner is reduced. Thus, contamination inside the apparatus will be caused by fogging on the image and toner splashing due to poorly charged toner. This will lead to the problem of reducing the service life of the development apparatus.
In the meantime, in the two-component developing system, toner is charged by triboelectric charging through mixture between toner and carrier. This results in a smaller stress and greater resistance to possible deterioration of toner. Further, a carrier for electrostatically charging the toner has a greater surface area, and therefore, is more impervious to contamination due to toner or external additive agent. Thus, a longer service life can be expected.
However, the carrier surface is also contaminated by the toner and external additive agent even when the two-component developer is used. The electrostatic charge of toner is reduced through a long-term use, and problems of fogging and toner splashing will arise. The service life cannot be said to be sufficiently long. A still longer service life should be ensured.
A technique of ensuring a prolonged service life of the two-component developer is disclosed in the Unexamined. Japanese Patent Application Publication No. S59-100471. It discloses a development apparatus wherein the carrier, together with toner or independently, is supplied little by little, and the deteriorated developer of reduced charging property is removed accordingly, whereby the carrier is replaced by a new one and hence the percentage of the deteriorated carrier is reduced. Since the carrier is replaced in this apparatus, reduction of the electrostatic charge of toner caused by carrier deterioration is kept to a predetermined level. This technique is efficient in ensuring prolonged service life.
The Unexamined Japanese Patent Application Publication No. 2003-215855 discloses a two-component developer made up of the toner and carrier with the opposite polarity particles having the polarity opposite to that of the toner externally added thereto, and a method of development using this developer. The opposite polarity particles of this development method serve as an abrasive powder and spacer particles. The carrier deterioration can be minimized by removing the spent matters of the carrier surface.
The Unexamined Japanese Patent Application Publication No. H9-185247 discloses a so-called hybrid development method wherein a latent image on the image carrier is developed using the toner-supporting member for carrying only the toner from the two-component developer. The hybrid development method has many characters that cannot be found in the conventional two-component developing system. For example, there is no brush mark on the image by a magnetic brush, excellent dot reproducibility and image uniformity is provided, and migration of the carrier to the image carrier (carrier consumption) does not occur due to the lack of direct contact between the image carrier and magnetic brush. In the hybrid development method, the toner is provided with triboelectric charging with the carrier, and therefore, maintenance of the charge-applying property of the carrier (toner charged triboelectrically by toner and carrier) is important to stabilize the toner charging property and ensure a long-term image quality.
However, the Unexamined Japanese Patent Application Publication No. S59-100471 involves cost and environment problems because a mechanism for collecting the ejected carrier is necessary, and the carrier is a consumable product. Further, a predetermined number of printing operations must be repeated until the percentages of the old and new carrier is stabilized, and the initial characteristics cannot always be maintained. Further, in the Unexamined Japanese Patent Application Publication No. 2003-215855 and Unexamined Japanese Patent Application Publication No. H9-185247, the carrier surface is contaminated by toner and finishing agent with the increasing number of printed sheets, and the charge-applying property of the carrier is reduced.